1. Field of the Invention
The present invention relates to a variable wavelength laser device, and in particular, relates to a variable wavelength laser device in which an external resonator is formed.
2. Background Art
Conventionally, variable wavelength laser devices are known in which a non-reflective film is formed on one end surface of a Fabry-Perot type laser diode, an element having wavelength selectivity is provided external to the laser diode on the non-reflective film, and feedback is applied to the laser diode, whereby an external resonator is formed in the region to the other end face. Such a variable wavelength laser device generates a laser when the gain conditions and phase conditions overcome the losses such as the reflection loss, scattering loss, and the like.
In this way, the laser light having a wavelength freely selected from among the light emitted from the laser diode generating the laser is selected by means of an element having wavelength selectivity, and this is fed back to the laser diode, and thereby, the wavelength is made variable.
FIGS. 4 and 5 are structural diagrams showing examples of conventional variable wavelength laser devices.
First, FIG. 4 will be described. In FIG. 4, reference 10 indicates the laser diode; a non-reflective film 10a is formed on one end surface thereof, while a low reflection film 10b is formed on the other end surface thereof. The reflectance of the low reflection film 10b is on the level of a few percent. A lens 12, a variable wavelength band pass filter 14, and a total reflection mirror 16 are formed in that order on the optical axis of laser diode 10 and on the side on which the non-reflective film 10a is formed.
Lens 12 converts the laser light emitted by the laser diode 10 to parallel rays. The variable wavelength band pass filter 14 has transmission characteristics such that only light having a specified wavelength passes therethrough, and this filter is disposed so as not to be perpendicular to the optical axis of the laser diode 10. Furthermore, the total reflection mirror 16 totally reflects light which is incident thereto. A resonator is formed by means of the total reflection mirror 16 and the end surface of laser diode 10 on which low reflection film 10b is formed.
Furthermore, a lens 18 and an optical fiber 20 are disposed on the optical axis of laser diode 10 at the side on which low reflection film 10b is formed. Lens 18 causes the laser light emitted from the end surface of laser diode 10 on which low reflection film 10b is formed to enter one end of the optical fiber 20. An emitter port is provided at the other end of optical fiber 20, and the laser light transmitted through optical fiber 20 is emitted from this port.
In the structure described above, when laser light is emitted from the laser diode 10, the laser light emitted from the end surface on which non-reflective film 10a is formed is converted to parallel rays by lens 12. When the converted parallel rays pass through the variable wavelength band pass filter 14, only the specified wavelength component is transmitted. The wavelength component which does not pass through variable wavelength band pass filter 14 is reflected; however, the variable wavelength band pass filter 14 is disposed so as not to be perpendicular to the optical axis of laser diode 10, so that there is no feedback of this light to laser diode 10.
The wavelength component which is transmitted through variable wavelength band pass filter 14 is reflected by total reflection mirror 16, and this again enters laser diode 10 via variable wavelength band pass filter 14 and lens 12. A portion of the laser light which enters into laser diode 10 is reflected by low reflection film 10b, and is fed back to laser diode 10, while the remainder of the light is emitted from non-reflective film 10b. In this way, resonance is achieved within the resonator formed by total reflection mirror 16 and low reflection film 10b.
The laser light omitted from the end surface on which low reflection film 10b is formed is focused by lens 18, and enters optical fiber 20 from one end thereof. The laser light which enters in this way is transmitted through optical fiber 20 and is emitted from emitter port 20a.
In the above structure, by making the transmission wavelength of the variable wavelength band pass filter 14 variable, the wavelength of the laser light fed back into laser diode 10 changes, and the resonance conditions change, and thereby, lasers can be generated at different wavelengths, so that it is also possible to change the wavelength of the laser light emitted from optical fiber 20a.
Next, the variable wavelength laser device shown in FIG. 5 will be explained; however, those parts which are identical to those of the variable wavelength laser device shown in FIG. 4 are given the same reference numbers, and an explanation thereof will be omitted here. The difference between the variable wavelength laser device shown in FIG. 5 and the variable wavelength laser device shown in FIG. 4 is that a grating 22 is provided in place of the variable wavelength pass filter 14 and the total reflection mirror 16 in FIG. 4.
Grating 22 has the characteristic of reflecting a specific wavelength component in a specified direction depending on the angle of incidence of the parallel rays which enter. Accordingly, by changing the angle thereof with respect to the incident parallel rays, it is possible to change the wavelength of the laser light fed back into the laser diode 10.
In the structure described above, the laser light emitted from the end surface on which non-reflective film 10a is formed is converted to parallel rays by lens 12, and this light is applied to grating 22. The wavelength of laser light reflected by grating 22 and which reenters laser diode 10 is determined by the angle formed by the parallel rays applied to grating 22 and the grating. A portion of the light which enters laser diode 10 is fed back into laser diode 10 at the end surface on which low reflection film 10b is formed, and the remainder of the light is emitted to the exterior.
In this way, a resonator is formed by grating 22 and the end surface on which low reflection film 10b is formed, and laser light having a wavelength matching the resonance conditions of the resonator is emitted. The laser light emitted from the end surface on which low reflection film 10b is formed is focused by lens 18, and is emitted from emitter port 20a via optical fiber 20. The wavelength of the laser light emitted from emitter port 20a may be changed by means of altering the angle formed by the grating 22 and the parallel rays converted by lens 12.
In the conventional variable wavelength laser device shown in FIGS. 4 and 5, in order to cause the generation of a laser, it is necessary to conduct a high ratio of light feedback to the active layers of the laser diode. Generally, the active layers formed on laser diode 10 have a cross-sectional area of approximately 1 micrometer.times.a few tens of nanometers in the plane perpendicular to the optical axis of laser diode 10. Accordingly, because this cross-sectional area is extremely small, it is necessary to position the optical members constituting the device with a high degree of precision.
With the cross-sectional area described above, even if the light is focused by a lens and applied, the coupling coefficients of the light do not become very large. Furthermore, in order to increase the coupling coefficients, conventionally, advanced optical axis adjustment technology was required for the arrangement of the optical parts and advanced fixation technology was required to increase the reliability, so that the structure of the parts also became complex, and this caused obstacles in terms of productivity.
Concretely, to explain based on the example shown in FIG. 4, in order to feed back the laser light emitted from laser diode 10 back into laser diode 10, it was necessary to position lens 12, variable wavelength band pass filter 14 and total reflection mirror 16 with a high degree of precision, and it was also necessary to adjust lens 18 and optical fiber 20.